1. Field of the Invention
The present invention relates to semiconductor devices. In particular, the invention relates to a method of increasing speed and operating frequency of field-effect transistors by increasing the average carrier velocity in field effect transistors.
2. Description of the Prior Art
The switching speed of field effect transistors is limited by the carrier velocity. In very short semiconductor devices electrons are expected to move ballistically, i.e. without any collisions with phonons or impurities. This may boost carrier velocity far beyond the carrier velocities in longer devices. Effective electron velocities as high as 3.25.times.10.sup.7 cm/s in a channel of a 0.08 micrometer gate-length planar-doped pseudomorphic AlGaAs/InGaAs/GaAs quantum-well High Electron Mobility Transistor (H/EMT) have been observed. This value is substantially higher than typical electron velocities in high-speed GaAs field-effect transistors (1-2.times.10.sup.7 cm/s) and somewhat larger than the peak velocity in InGaAs (approximately 2.8.times.10.sup.7 cm/s).
Nevertheless, the advantages of ballistic and/or overshoot transport are diminished in short-channel field-effect transistors compared to vertical device structures, such as a Hot Electron Transistor or a Planar-Doped Transistor. In these vertical device structures, electrons enter the active region with considerable initial velocities. Consequently, the overall transient time is decreased thus improving the probability of ballistic transport of the electrons.
In a field effect transistor (i.e. a lateral device), on the other hand, electrons initially enter the channel with a low velocity. The electrons are gradually accelerated toward the drain due to the electric field in the field effect transistor. As numerical simulations of high-speed field effect transistors clearly indicate, the maximum electron drift velocity is reached near the drain (See, for example, A. Cappy et. al., "Comparative Potential Performance of Si, GaAs, GaInAs, InAs Submicro-meter-gate FETs", IEEE Trans. Electron Devices, Vol. ED-27, p. 2158 (1980)). The electrons move very fast in the region near the drain but relatively slow in the region near the source due to the low initial velocity of the electrons as they enter the channel. The electrons are thus more likely to experience collisions limiting their acceleration in the region near the source. The device speed is determined by the overall transit time under the gate. The device speed is heavily affected by the relatively slow electron drift velocity in the channel near the source region.
Despite a high level of interest in high speed semiconductor devices, and considerable theoretical and experimental efforts directed to the use of ballistic transport of electrons in field effect transistors and other means of increasing transistor speed, development of a high speed GaAs field effect transistor with electron velocities approaching that of vertically structured devices has yet to be achieved.